Chemical mechanical polishing (CMP) is a process for very finely polishing surfaces under precisely controlled conditions. In applications such as polishing wafers and integrated circuits, the process is used to remove a few angstroms of material from an integrated circuit layer, removing a precise thickness from the surface and leaving a perfectly flat surface. The surface to be polished may be comprised of many materials, including various metals and silicates.
To perform chemical mechanical polishing, a slurry comprising a suitable abrasive, a chemical agent which enhances the abrasion process, and water is pumped onto a set of polishing pads. The polishing pads are rotated over the surface requiring polishing. The amount of polishing (the thickness removed and the flatness of the finished surface) is controlled by controlling the time spent polishing, the distribution of abrasives in the slurry, the amount of slurry pumped into the polishing pads, and the slurry composition (and other parameters). It is therefore important to control each of these parameters in order to get a predictable and reliable result from the polishing process. In particular, unreliable slurry flow rates cause fluctuations in removal rates and a large number of unacceptable finished wafers or circuits.
The slurry used for polishing is sensitive to degradation by the components in the slurry flow path. Whenever the slurry is subject to shear forces created by intrusive mechanical components such as pump impellers, pressure gauge taps, or flow meter vanes, its abrasive particles have tendency to agglomerate. This agglomeration results in uneven polishing, scratching, and other defects in the polished surface. Accordingly, peristaltic pumps are used to pump the slurry because these pumps have no impellers which impart shear forces to the slurry. However, flow rate is often measured with vaned flow meters or other intrusive and shear creating flow meters which rely of the insertion of physical structures into the slurry flow (any agglomeration is tolerated, and results in lower reliability and yield of the system).